In the recent years, biometrics, and especially fingerprint sensors, have become common for the purpose of verifying the identity of a person, e.g., at immigration control and at airports, as well as with personal devices, such as laptops, mobile phones, tabs, pads, etc. The present solutions still have a number of disadvantages. Fingerprint sensors used in airports and immigration control are large and too expensive for many applications, and smaller sensors seen in portable devices are often silicon based solutions with limited robustness, design flexibility, and challenging electronic interconnections. Traditional silicon production techniques for such sensors often result in solutions for electrical interconnection features interfering with the physical finger interface of the device. Recessed mounting of the sensor in a consumer application is often implemented to improve these shortcomings, but may not be the optimal solution both with respect to esthetical design and protection from dirt and moisture. Sensor size, both volume and area, along with the rigid properties of silicon, significantly limits the feasibility of integrating fingerprint devices in thin and flexible applications such as smartcards.
A fingerprint sensor which may be flush mounted in the same plane as the surface of the product it is mounted into is described in U.S. Pat. No. 7,251,351, in which a set of first electrodes/sensor elements is positioned on one side of an insulating substrate provided with through-substrate-via conductors. The substrate may be made of glass, ceramics or other insulating materials. In international patent application WO2011/080262, a similar solution is discussed based on a flexible material for low cost production. A known fingerprint sensor is also described in US2009/0252385 and U.S. Pat. No. 7,099,496 where the characteristics of the finger surface are measured by the effect of the field between a number of wire ends and an electrode extending at a distance from the wire ends. The electrodes may be position over or under a dielectric material. This solution has a limited resolution as it depends on the radial, fringing field extending from the wire ends to the perpendicular drive electrode and also requires a high accuracy in positioning of the electrodes. Another example of the known art is presented in U.S. Pat. No. 8,224,044 where the circuitry is positioned on the opposite side of the substrate from the finger surface. This provides for simple manufacturing but at the cost of resolution.